Assessment of Eurasian lynx reintroduction success and mortality risk in north-west Poland

Despite recent local reintroductions of the Eurasian lynx Lynx lynx in central and north-eastern Poland, the increase in its population was not followed by its westward expansion. To address this problem and restore the lynx population in north-western Poland, 61 captive-born individuals of Baltic population origin were released in the province of Western Pomerania in 2019–2021. Prior to their release, all the animals underwent an adaptation training phase. They were then set free according to a hard-release protocol and subsequently monitored by means of GPS telemetry. In order to assess the short-term reintroduction success, the survival and causes of death of the released individuals were studied as a function of sex, age, training time, and release time and place. The overall survival rate was 71.15%, the median survival time 202 days. Most mortality, due to environmental factors, i.e. scabies (> 200 days) or traffic collisions (< 200 days), was recorded during the first 300 days following release. Age, year of release and training time were significantly related to survival, indicating that the older the lynx was when released, the better its survival changes. In contrast, the longer the training time, the poorer were the chances of survival. There was no evidence of any effect of sex, month of release or place of release. Based on these results, recommendations were made for the planning of further releases and measures to manage the restored population.

Assuming that the fate of lynx from which telemetry data are no longer received is known, this parameter would be 74.58% for all released individuals (N = 59), 69.7% for males (N = 33) and 80.77% for females (N = 26) if these animals were still alive, or 62.71% for all animals (N = 59), 54.55% for males (N = 33) and 73.08% for females (N = 26) if these animals were dead. The median survival time (N = 15) was 202 days (males-186.5 days, females-202 days). No significant differences were found between the sexes (Mann-Whitney U-test for comparisons of median values, P = 0.95), but there was much greater variability in life expectancy in the wild for females (Fig. 2).
Most mortality was recorded during the first 300 days following release (Figs. 3 and 4). The overall probability of surviving beyond three, 12, 18, 24 and 30 months was equal to 92.2%, 73.8%, 70.1%, 65.4% and 60%, respectively (N = 52). The survival time for males declined to a plateau of about 60% in 680 days, while the fall in female survival was slower, declining to a plateau of about 55% in 740 days.
Taking into account the age of sexual maturity (3 years for males and 2 years for females 5 ), the released animals survived in the wild for 1.65 mating periods on average, during which they had the opportunity to mate and breed (N = 20). For males (N = 8) this average was 1.75, and for females (N = 12) 1.58 mating periods (Fig. 5).
Cox proportional hazard analysis. According to the Cox proportional hazard regression analysis (Table 2), the following variables were significantly related to survival-age at release (P = 0.0002, Hazard ratio = 0.718), training time (P = 0.0003, Hazard ratio = 1.011), defined as the number of days spent in adaptation enclosure and release centre, and year of release (P = 0.0117, Hazard ratio = 0.197) (N = 50). No evidence was found for any effect of sex, month of release or place of release.
Causes of death. The cause of death could not be determined in 6 out of 15 cases of mortality among the released animals recorded in 2019-2021 (Fig. 6). The majority of deaths (55.6%) were attributed to environmental factors, i.e. scabies (sarcoptic mange), and four (44.4%) were caused by humans (animals hit by a road vehicle or, in two cases, by a train).
There are indications that one of the animals may have been killed by a poacher, one may have died of an unknown infectious disease and one may have been killed by a predator (possibly another lynx). However, there is no hard evidence to substantiate these suspicions.
There was a statistically significant difference in the life expectancy of lynx in the wild from the day of release to the day of confirmed death caused by a collision with a road vehicle or train and by other causes of death, most probably infection with environmental factors, like scabies (Mann-Whitney U-test, P = 0.015). This indicates that the limiting value to which death was caused by road and rail collisions is 200 days after release. After this period, environmental factors (scabies) was the main cause of death of animals in the wild (Fig. 7).

Discussion
The success of a long-term reintroduction programme is assured when a viable, sustainable, ecologically and genetically functional population has been restored in the wild. The effectiveness of measures implemented to achieve this goal can be assessed with the aid of post-release monitoring, defined by indicators of short-term reintroduction success, i.e. survival of released animals and breeding by released animals 30 . The very effectiveness of these measures depends on the animals' hunting efficiency (adaptation to life in the wild conditioned by pre-release training) and the avoidance of human-carnivore conflicts (appropriate reintroduction programme design and conflict management in the later stages), with subsequent mating that ends in parturition and the rearing of kittens (reproductive success) 31,32 .  www.nature.com/scientificreports/ On average, seven out of every ten lynx released in 2019-2021 in north-western Poland, whose fate is known, were alive as of 30 September 2021. This tallies with survival rates between 63 and 95% for this species reported by other authors (as revised by Franz and Romanowski 8 ). The mean survival time (from release to recorded death) of animals released during the above-mentioned three-year period was 275 days (5-870 days). However, one male-still alive-has so far spent 981 days in the wild. This mean value is comparable with that for the reintroduced population in the Kampinos National Park in Poland (272 days) 8 . The average age of lynx released in north-western Poland is, as of 30 September 2021, slightly above 37 months. The Eurasian lynx is reported to live up to 17 years in the wild, but the average age of resident animals in a population is only about 4-5 years 5 . At the same time, it is estimated that under natural conditions, only 24% of lynx lived for three years in the Białowieża Forest (Poland) 33 . A study carried out in the Bohemian Forest, on the Czech-German border, indicated that individuals up to 4 years of age made up 64% of the entire population and that the mean generation time for resident reproducing females was 2.64 years 34 . Meanwhile, the probability of reaching the age of 2.5 years for lynx reintroduced to north-western Poland was 51.1%. The sex of the animals had no effect on the probability of survival, which suggests no effect of sexual dimorphism. Differences in the probabilities of male and female Table 1. Characteristics of Eurasian lynx reintroduced in north-western Poland in January 2019-July 2021 (age-as of 30 September 2021 for live animals; survival time-number of days from release until death was confirmed or until 30 September 2021 for live animals; fate (status as of 30 September 2021): C-caught and placed in an enclosure, A-alive, L-a female that had a litter, D-dead, with cause of death marked in parentheses as unknown (U), environmental factors/scabies (S) or road collision (C), U-status unknown; n/a-not applicable; the animals caught soon after release and placed in the breeding centre are marked in bold; the animals caught soon after release and re-released in another location are marked in italics).      35 ) and without sexual dimorphism (e.g. swift fox Vulpes velox 36 ). When assessing the reproductive success of the reintroduced lynx, one has to assume that the number of mating seasons during which they could reproduce is comparable with the number of litters, i.e. mating seasons during which animals successfully reproduced. In the case of the lynxes covered by the "Return of the Lynx to north-western Poland" project, ten litters were recorded in 2019-2021, which translates into 30.3% of the chances of mating and successful breeding. On the other hand, the fact that eight females had a litter at least once means that 80% of the sexually mature reintroduced animals (83.3% of females) reproduced at least once in the wild in 2019-2021. For comparison, only 35% of the released females in the population reintroduced to the Kampinos National Park were found to be breeding 37 .
The effect of the covariates on the survival curve was shown to be strong and significant for the age at release and the time spent in the adaptation enclosure and release centre (training time), while year of release had less significance. The hazard ratios of covariates, explaining the multiplicative effects on the ratio of death, indicate Table 2. Results of the Cox proportional hazard regression analysis (N = 50; the value of estimate means the covariate increases the hazard of death/decreases the survival time; HR 95 and HR 95 indicate the lower and upper 95% confidence limits, respectively). The significance of the proposed model was evidenced by the Likelihood ratio test (Statistic = 43.6, df = 6, P = 0.000), the Wald test (Statistic = 23.61, df = 6, P = 0.000), and the Score (logrank) test (Statistic = 29.31, df = 6, P = 0.000).    www.nature.com/scientificreports/ in particular that being older when released augurs well for survival in the wild, whereas a longer training time is linked with poor survival prospects. The results therefore suggest that the post-release survival of younger reintroduced individuals is lower, probably because they have a lower potential for coping with the stress associated with translocation than older ones 38 . The relationship between the survival rate of reintroduced animals and the year in which they were released is governed by stochastic meteorological factors (obviously independent of humans) and therefore cannot be mitigated in the hard-release scenario. To overcome interannual variation as regards climate and infrequently occurring natural disturbances, IUCN recommends releasing individuals over several years 28 . It should also be noted that the number of releases in a given year may distort the interpretation of the results, as more releases in later years translates directly into fewer days spent in the wild by late-released animals. There is no evidence of any effect of sex, month of release and the place where the animals were released on the life expectancy of reintroduced lynx in the wild. Both, sex of released individuals and the place of release (analysed in terms of linear distance dependence on the training centre) makes no statistically significant contribution to the difference in the hazard ratio after adjusting for age, sex, month, year and training time. In the case of lynx reintroduced to north-western Poland, releasing the animals in the earlier months of the year did not appear to favour their greater survival. Such a dependence was demonstrated by the results of an experiment demonstrating that the survival of released least weasels Mustela nivalis improved when releases took place during a season with abundant food resources 39 . The Canada lynx Lynx canadensis reintroduction protocol also recommends releasing them in the spring months, so to ensure the highest annual abundance of prey 40 . The insufficient amount of data probably determined the lack of significance of survival in relation to a place of release, determined by the distance from the training centre. Despite some tendencies reported by practitioners, this issue requires further investigation.
According to Maran et al. 35 the following factors potentially influence the survival of released animals: biological variables (age, sex), pre-release factors (maintenance conditions, pre-conditioning, experience with humans), release methodology (hard or soft release), release site characteristics (availability of suitable shelter and habitat, abundance of predators, availability of food resources, level of disturbance). The biological variables include the genotype and thus the origin of the animals. The use of wild animals in mammalian carnivore restoration is sometimes claimed to be more successful than the use of captive-born ones 38,39 . In contrast to the above statement, the use of captive-breed Eurasian lynx is recommended for reintroductions 41 . The lynx recruited for the reintroduction programme in north-western Poland were of captive origin and belonged to the Baltic population. Two aspects favour this approach. Firstly, the reintroduction carried out in the Harz Mountains (central Germany) demonstrated that zoo-born individuals were able to adapt to life in the wild, and secondly that a considerable advantage of captive-bred animals over wild ones is the possibility of conditioning their genetic profile and health status before release 41 .
An analysis of previous restitutions of mammalian carnivores points out that far too little attention is paid to sociological and organizational variables 20,42 . The latter include pre-release treatment, behavioural conditioning (training) and pre-release housing conditions (active adaptive management) 28,35,43 . As it turns out, the previously mentioned claim regarding the greater survival chances of reintroduced wild-born animals is based largely on the fact that they are better able to tolerate translocation stress 38 . At the same time, many authors emphasize the possibility of a positive impact of appropriate pre-release training (learning effect) on shaping the behaviour of individuals intended for reintroduction (e.g. inducing development of "natural" behaviours, familiarity with the natural habitat and avoidance of danger), increasing their post-release chances of survival and life span 8,28,30,38,44,45 .
The results indicate that pre-release training (enclosure effect) significantly influences the survival of reintroduced animals, but in a rather non-obvious way: remaining too long in the adaptation enclosure and the release centre is negatively correlated with the number of days that the released animal survives in the wild. While the training itself is undoubtedly of great importance in adapting to post-release life in the wild, too much of it can produce a side effect in the form of pre-release habituation to captivity, to humans and, generally, to stimuli that may pose a danger after release, as has been shown, for example, in the case of released swift foxes in North America 44 .
The mortality of reintroduced animals is a derivative of biological and sociological variables. It is commonly believed that most lynx mortalities in Europe are caused by humans and result from recreational harvesting, poaching and vehicle collisions 5,8,20,46,47 . In the reintroduced population in north-western Poland, the only confirmed causes of animal death were environmental factors (scabies) and traffic collisions. Moreover, the numbers of the former, i.e. death from a natural cause, were slightly higher. Likewise, in a strictly protected area in the Bohemian Forest, lynx were not found to be strongly affected by human-related mortality 34 . By contrast, a study in the Swiss Alps and the Jura Mountains (Switzerland) showed that natural causes accounted for 44.9% of all established causes of death in this region and that infectious diseases accounted for 26.5% of them. Scabies alone accounted for only 6.1% of deaths from a known cause 48 . Sarcoptic mange, caused by Sarcoptes scabiei, is a highly contagious epizootic skin parasitosis, distributed worldwide and observed most frequently in mammals 49 . Occasional cases of scabies as the cause of death of lynx in Europe have been reported from Norway 50 , Sweden 51 , Switzerland 52 and Poland 49 . The source of infection in lynx is likely to be the red fox Vulpes vulpes, in which case S. scabiei must be endemic in some parts of Europe 52,53 . The recently upsurge in reports of outbreaks in new locations suggest that this parasite is expanding across the continent 53 .
One of the most interesting conclusions from the results of the present study is the clear distinction in life expectancy of reintroduced lynx that died as a result of traffic collisions and as a result of environmental factors, i.e. scabies infection. These results indicate that the former is the most serious cause of death in newly released animals (up to 200 days in the wild), while the latter represents the main known cause of mortality in animals surviving more than 200 days in the wild. This fact may be of great practical importance in terms of modifying pre-release training, but the topic needs to be further explored. Even if "quick learners" cannot avoid the natural, objective threat of scabies, their ability to avoid traffic accidents does enhance their chances of survival in the wild. Numbers of observations being limited, it is recommended that further data should be collected on lynx www.nature.com/scientificreports/ mortality and that analyses in this respect should be continued. It should also be borne in mind that appropriate pre-release training to avoid danger can have a considerable effect on lynx survival, as well as the implementation of conservation measures improving the safety of wild animals vis-à-vis road and rail infrastructure, such as under-and overpasses, reduced speed zones, and fencing 8 . The effectiveness of such an approach has been demonstrated in reducing the effect of road mortality on the Iberian lynx Lynx pardinus in Spain 54 , and the need for vehicle strike mitigation in the case of the bobcat Lynx rufus in Ohio (U.S.A.) 55 .
The results obtained for north-western Poland (in particular, the lack of significant human-lynx conflict cases revealed by the three-year post-release monitoring) augur well for the possible coexistence between lynx and humans, also reported from other parts of Europe, thus indicating that they can share the same landscape 7,56-58 . Cases of felids capable of persisting even in heavily human-modified landscapes (Panthera tigris, Panthera pardus, Puma concolor) were reviewed by Bouyer et al. 56 .
In conclusion, although the present study was to some extent limited by the sample size, the short-term results of the Eurasian lynx reintroduction programme in north-western Poland provide the most comprehensive and substantial scientific basis to date for planning the restoration of this felid in the Southern Baltic Lake District, and also inform future releases of this felid in similar conditions. The following conclusions and recommendations can be formulated, based on the results obtained and literature review: 1. In order to ensure better survival, it is preferable that mature animals be released. 2. The pre-release training of animals should not last too long and should focus on arousing fear in them of road vehicles and trains. 3. As the first 300 days after release are critical for the release operation (the period with the highest mortality), any interventions supporting the survival of the released individuals should focus on this period. 4. Telemetry collars make it possible to precisely monitor the fate of the released animals, thus enabling a comprehensive assessment of the short-term effectiveness of the reintroduction measures. 5. Monitoring of the reproduction, movements and behaviour of the released animals should be continued (telemetry collars replacement and maintenance) in order to assess the long-term effectiveness of the reintroduction project.
It should be also noted, that despite the promising results regarding the survival and reproduction rate of the released animals, it seems advisable to continue the reintroduction project of the Eurasian lynx in north-western Poland at least until the number of kittens born in the wild begins to compensate for the losses due to sub-adult and adult mortality. Termination of the project at the present stage would mean the gradual disappearance of the population hitherto created, as shown by the experience from central Germany 19 . However, further research into the reproductive success of the reintroduced population is necessary.

Methods
Study area. The area selected for reintroduction (Ińsko Lakeland, Drawsko Military Compound and Mirosławiec Forestry; Fig. 8) is situated in the Southern Baltic Lakeland (province of Western Pomerania). It is a young post-glacial landscape, well forested (44% of the area is woodland), with minimal fragmentation (a dense network of forested ecological corridors) and large numbers of roe and red deer (720 kg/km 2 ) 26 . Tree stands more than 100 years old account for c. 10% of the forest area 59 . The dominant tree species is pine (nearly 60% of all the forest trees), and the amount of harvested timber is 572.1 m 3 per 100 ha of forest 59 . The average annual air temperature is 10.8 °C, with extremes of − 30 and 37.8 °C. The total annual precipitation is 705 mm, and the averages of wind speed, insolation and cloudiness are 3.5 m/s, 1.976 h and 4.9 octants, respectively 59 . 21.8% of the area consists of land subject to various forms of legal protection (national parks, nature reserves, landscape parks, etc.) 59 . The region is predominantly rural (93.8%), with an average population density of 74 inhabitants per km 2 . At the same time, 68.4% of the population lives in urban areas 59 . The densities of main roads, secondary roads and railways are 0.08, 0.10 and 0.05 km/km 2 , respectively 26,59 . There are 0.78 road vehicles per inhabitant 59 . It is estimated that the project area can host at least 80 lynx, guaranteeing a survival of 100 years at the level of 57% if isolated 26,60 . Origin of the released animals. The Eurasian lynx reintroduction programme in north-western Poland involved 61 captive-born individuals originating from the Baltic population, imported from European zoos and enclosures (not associated with the European Association of Zoos and Aquaria, EAZA), and two animals born in the Dłusko breeding centre (Table 1) 26,27 . Individuals kept in this breeding centre derive from the EAZA breeding programme, for which a European studbook (ESB) for Northern lynx L. l. lynx was established. Only individuals that genetically did not differ significantly from wild lynx within the Baltic population were included in the breeding and reintroduction programme (each individual was subjected to genetic testing prior to release). www.nature.com/scientificreports/ observation during feeding), they were assessed for the next pre-release steps. The period of adaptation was individually adapted to the behaviour of each animal. Most lynx required only a short, 1-2 week training period before release, but some others needed more training and stayed longer in the semi-natural environment of the adaptation enclosure in Dłusko. The Dłusko facility has six enclosures from 0.5 to 0.8 ha in size, located within a fenced area (enclosure) of 90 ha. Careful observation made it possible to select lynx for breeding, which were then moved to the breeding centre in Jabłonowo (facility of the West Pomeranian Nature Society) because of their low degree of anthropophobia. Secretive animals, exhibiting shy, cautious behaviour and avoiding contact with humans, were intended for release and trained for hunting wild prey. It should be noted, however, that the lynx's strong hunting instinct does not require reinforcement and the main behaviours to be enhanced were avoidance of humans and arousing fear of humans 26,27 . Their contacts with keepers were thus reduced to a minimum and restricted to unavoidable husbandry. During the entire pre-release training period, the animals were fed exclusively with the meat of their natural prey. Lynx intended for release were moved to the outside enclosures of the release centres in Mirosławiec and Drawsko (situated in forests), where they were initially fed, after which feeding was limited and the lynx were left outside the enclosure, usually after 1 week. During adaptation period animals were kept in natural enclosures, with trees, bushes, fallen trees, branches, stones and several hiding places (in the case of the adaptation enclosures in Dłusko, and release centres in Mirosławiec and Drawsko; Fig. 8). Related individuals were released in different locations, except for mothers and their kittens. Releases took place in different months throughout the year, in forested areas away from human settlements, and followed a hard-release protocol. The age, sex, release date and place, and time spent in the adaptation enclosure and release centre were recorded for every individual released. Due consideration was given to animal welfare at all stages of the reintroduction process.
Radio-tracking and post-release monitoring. All released lynx were tracked using GPS telemetry.
Each released individual was fitted with a GPS/GPRS/VHF telemetry collar weighing from 280 to 320 g, matched to the animal's size. The collars, manufactured according to an original design by the West Pomeranian Nature Society, have a GPS locator that records a lynx's position every 3 h, a GPRS/GSM communication module that www.nature.com/scientificreports/ transmits data to telemetry servers twice a day, and a continuously operative VHF transmitter with an independent power supply, enabling the animal in question to be sought in the field. The maximum estimated working time of the GPS module is 15 months, whereas the VHF transmitter can work for up to 3 years. The collar is equipped with a mechanical dropper, which allows a lynx to lose its collar after about 2-2.5 years. Collars with dead batteries were replaced with new ones. All the antennas are concealed in the telemetry device case and collar strap, as a result of which the collar has no protruding components. With telemetry data one can determine with a high degree of probability the sites where hunting has been successful. In the first period after release, all the sites where prey was caught were checked to assess the degree of independence of the released animals and the type of catch. In the following months, all lynx that exhibited atypical movements or were inactive (stationary) for a suspiciously long time were tracked and monitored in the field. Where necessary, on site observations enabled the cause of unusual behaviour or death to be discovered. Individuals lost to radio-tracking and not subsequently recaptured were assumed to have died (designated as "unknown status" and excluded from further analysis). Post-mortem examination was conducted by a qualified veterinarian, in accordance with routine veterinary medicine protocols and post-mortem procedures for wildlife veterinarians 61 , on all dead lynxes found in the field, in order to determine a proximate cause of death. The examination included a systematic necropsy, except in cases where obvious traumatic traffic accidents caused the death (X-ray imaging performed if needed), and was focused on the following etiological factors: congenital, infectious, nutritional, toxicological, traumatic, other potential reasons founded on generalized etiologic principles (politraumatic events, multi-raility of limbs, fractures, traumatic haemorrhage). Scabies was diagnosed based on a visual observation of skin lesions and confirmed by detecting the mites in skin scrapings with microscopy. Mites were collected by scraping affected skin areas with a sterile surgical blade, following by placing samples on a glass plate, dripped with liquid paraffin and consequently covered with a glass cover and then examined microscopically under 100-400 × magnification. Mites were morphologically identified as S. scabiei using a diagnostic reference 62 . The submission of tissue samples for further diagnostics was based on necropsy findings when additional insight was warranted to confirm or determine the cause of death, and included microbiological (bacteriological culture), biochemical (c-reactive protein) and toxicological (coumarin and coumarin derivatives, ethylene glycol, strychnine, organophosphorus insecticides, bromethalin, petroleum, toxin alkaloids, cicutotoxin, organochlorine insecticides and mycotoxins) analyses commissioned to reference laboratories. During the necropsy examination all samples were taken in rigorous asepsis conditions and transported for the further analyses as soon as possible. All post-mortem examinations were documented (descriptively and photographically).

Survival analyses.
The survival rate was calculated as the percentage of individuals remaining alive as at 30 September 2021 in the pool of animals released into the wild and not caught and placed in the breeding centre, and whose fate in the wild was known (N = 52). Lynx whose fate in the wild was not known were included in the analysis of hypothetical survival rates calculated on the assumption that these animals were alive or dead (N = 59). Life expectancy was calculated for those animals whose death was recorded after release into the wild (N = 15), to exclude the effect lowering the value of this indicator, resulting from the analysis of the situation for the selected time point (30 September 2021), after which a significant percentage of the released animals are still alive. In addition, the average number of mating periods survived in the wild by reintroduced sexually mature individuals was calculated.
Overall (including plot as a function of place and year of release) and sex-dependent cumulative survival probability curves were plotted by the Kaplan-Meier method 63 and visualized using the 'Survival' v. 3.2-13 64 , 'Survminer' v.0.4.9 65 and 'SurvMisc' v. 0.5.5 66 packages for R 67 . Two males caught after being released and placed permanently in the breeding centre (Nos. 1 and 2 in Table 1) were excluded from the survival analyses (N = 59).
Statistica v.13.0 PL software (TIBCO Software Inc.) was used to assess the statistical significance of the difference in overall survival times between males and females (Mann-Whitney U-test for comparisons of median values) 62 and differences in life expectancy of lynx in the wild from the day of release to the day of confirmed death caused by a traffic collision or by environmental factors, i.e. scabies (Mann-Whitney U-test) 68 . Statistical significance was at P ≤ 0.05. 69,70 was applied to investigate the association between the survival time (life expectancy) of released individuals and the following predictor variables: age (at release), sex, month of release, year of release, training time and place of release. For this, the 'Survival' v. 3.2-13 R package was used 64 . The significance of the proposed model was assessed by the Likelihood ratio test, the Wald test and the Score (logrank) test [71][72][73] . Two males placed permanently in the breeding centre (Nos. 1 and 2 in Table 1), two females caught after release and re-released in other locations (Nos. 11 and 28 in Table 1), as well as seven individuals of unknown status (Nos. 4, 25, 30, 37, 39, 41 and 53 in Table 1) were excluded from the analysis of the above-mentioned co-variables. All data used in the Cox analysis concerning live individuals at the time of closing the analysis were treated as a right-censored data. Sex data, of a binary nature, were coded as: males = 1, females = 2. In the case of data on a release site, the model uses the code 1 for animals released in an immediate vicinity of the training site, while the following numbers encode the two remaining release sites, the sequence of which is determined by an increasing distance to the training site: adaptation enclosure in Dłusko = 1, release centre in the Drawsko Forest District = 2, release centre in the Mirosławiec Forest District = 3 (Fig. 8).